I used one shortly on a Macbook in the past (I think it was from Kensington, but I am not sure). The problem was that the reduction of the viewing angle made spontaneously sitting down with a colleague to discuss something showing it on the screen a little bit tedious... even alone you had to sit more or less centered in front of the screen otherwise the brightness went down quite a bit.
Of course that is intended to protect against peeking of the content on the display by an observer, but at the end it represented to much hassle compared to what I gained. Of course YMMV.